ABSTRACT: The morphology of branching corals effectively blocks impinging currents. Weak flows in the colonys inner sections can hinder some of the corals basic physiological functions. At night, high respiration by the coral and the absence of photosynthesis by the symbiotic algae sometimes generate severe hypoxia near coral branches. Nevertheless, branching corals thrive in flow-protected areas. Many of them host resident fish that seek shelter between their branches during the night. In situ observations using an infrared-sensitive video camera revealed that, unlike crevice-dwelling fishes, which sleep motionless, fish (Dascyllus marginatus, D. aruanus, and Chromis viridis) that spend the night in living hard corals (Stylophora pistillata, Acropora spp., Pocillopora spp., and Seriatopora spp.) exhibit a unique sleep-swimming mode that is characterized by energetic, high-frequency fin motions. In the reef, the presence of fish in the coral S. pistillata increased gypsum dissolution and, hence, mass transfer, by ~17% compared with colonies without fish. The extent to which the fish enhanced water replenishment in the inner zone of the coral was assessed in the laboratory using oxygen measurements. In the absence of fish, oxygen levels near the coral tissues (~1 mm) rapidly declined, reaching 10-30% of ambient levels, which was much lower than the nearly steady levels of 60-80% when sleep-swimming fish were present. Individual sleep-swimming fish were spatially separated across the coral, so that most of the corals inner zone was frequently ventilated. This common, yet so-far overlooked mutualistic relationship is unique by the virtue of its effect, the active modulation of hydrodynamic conditions, and its operation during the sleep of the active animal. The modulation of hydrodynamic conditions by the fish can mitigate flow limitation on the growth and survival of branching corals.

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